1. Field of Invention
This invention relates to the art of electrochemical cells and more particularly to a method for manufacturing a cathode component for use in an electrochemical cell from a free-standing structure of cathode active material. Preferably, the free-standing structure of cathode active material comprises a metal-containing material that is intercalatable with metal ions from the anode after being press laminated onto a current collector.
2. Prior Art
It is known to form cathodes of metal oxides and mixed metal oxides, such as silver vanadium oxide cathode material, by a pressing process that requires powder mix charges to be manually spread into press fixture cavities. The prior art process is illustrated in the block diagram flow chart in FIG. 1 and begins with introducing the cathode material as a dry granular admixture comprising conductive diluents and a suitable binder material into press platens or fixtures 12. The loaded fixtures are then placed in a hydraulic press which compresses the cathode admixture into cathodes plates for use in an electrochemical cell, as is known to those of ordinary skill in the art.
More particularly, receptacle cups for the cathode admixture are first weighed on a tare scale 14 and then filled with the dry cathode material 16. Two charges of the cathode material 18 are weighed in this manner and a first weighed charge is loaded into a bottom press fixture 20. This requires an operator to chop and spread the cathode material to form an even consistency throughout the fixture 22. A suitable current collector is placed on top of the dry powder cathode admixture centered in the fixture 24.
A top fixture is then positioned on the bottom fixture, ready to receive a second weighed charge of the cathode material 28. Again, the second charge must be chopped and spread evenly in the top fixture 30 to ensure uniform distribution therein. A plug is positioned on top of the second cathode charge to close the fixture assembly 32 and the assembly is placed into the hydraulic press 34. The compressed dry charges of cathode material, having the intermediate current collector sandwiched therebetween 36, form a cathode component suitable for use in an electrochemical cell.
After pressing is completed, the fixture assembly is removed from the hydraulic press 38 and separated from the cathode component. The newly pressed cathode component is then placed in a collection tray 40 ready for assembly into a high energy density electrochemical cell, as is well known to those of ordinary skill in the art.
There are several problems associated with this prior art manufacturing process including dust generated as a result of handling the dry cathode mix. This cathode dust being airborne represents material loss which may need to be replenished. Additionally, the step of chopping and spreading the dry cathode material admixture loaded into the fixture cavities adds significantly to the process time required to make a cathode component. If this step is not carried out properly, the resulting cathode plate will have a non-uniform density or unit weight, which can detract from the performance of the electrochemical cell.
U.S. Pat. No. 4,956,247 to Miyazaki et al. describes a process for making a positive electrode for use in a secondary electrochemical cell comprising a mixture of a metal oxide or a mixed metal oxide material having an average particles size of up to 3 .mu.m, a conductive agent such as acetylene black, and a binder such as a powder fluoro-resin. These ingredients are kneaded into an admixture with an organic solvent to form a paste that is subsequently formed into a cathode sheet by moving the paste through a roll mill followed by drying. A coupon is cut out of the resulting sheet and bonded to a stainless steel plate by means of a conductive adhesive containing carbon to thereby form the positive electrode.
This prior art process requires the use of the conductive adhesive to bond the cathode sheet material to the current collector. The adhesive may detract from the cell performance and the adhesive represents an additional manufacturing step that is not needed in the process of the present invention.
In the present invention, a cathode current collection is laminated between at least one shaped cathode structure pressed on each side of the current collector. This is accomplished without the use of conductive adhesives so that in the present invention the cathode structures are in direct contract with the current collector. The compression of the cathode structures onto the current collector provides the needed electrical contact rather than the use of an adhesive.
U.S. Pat. No. 4,536,618 to Shia discloses a sheet process for fluorinated carbonaceous cathode materials. Carbonaceous cathode active materials are organic materials, and as such, they are relatively soft in comparison to the metal-containing cathode active materials of the present invention. For that reason, it is critically necessary for Shia to mix the carbonaceous cathode active material with a volatile, substantially nonpolymeric, pore forming solvent to form the paste. It is the step of removing the solvent, by evaporation, from the paste that imparts the necessary porosity to the cathode sheet. More accurately, the removed solvent provides permeability to the cathode sheet that enables the metal ions produced by oxidation of the anode to intercalate therein. However, in Shia, the cathode sheet material is not intended to be incorporated into a cathode component by being pressed onto a current collector. Such a final pressing step, after the pore forming solvent has been removed from the cathode sheet, would destroy the permeability of the cathode sheet. The relatively soft carbonaceous material would tend to compact to a tight, impermeable mass not intercalatable with metal ions produced by the anode.
U.S. Pat. No. 4,161,063 to Goebel et al. discloses a method of preparing a cathode structure for use in an electrochemical cell. The preferred composition of the cathode material comprises up to 30 weight percent of graphite, 65 to 99 weight percent of carbon black and 1 to 10 percent of a fluorocarbon polymer mixed with a 50% isopropyl solution in water. The resulting dough is extruded in a spaghetti-like form that is chopped into a plurality of discrete porous conglomerates. The conglomerates are then pressed onto both sides of the substrate serving as a current collector to form the cathode structure. In the Goebel et al. method for making a cathode for an electrochemical cell, the step of chopping is critically important in order to provide the resulting cathode component with the requisite "fluid conducting channels" so that the metal ions produced by the anode are able to intercalate therein. Without these channels, the resulting cathode component would not be suitable for use as an electrode in the electrochemical cell.
In the present invention, there is no need for the solvent material to act as a pore-forming agent or for the cathode structure to be chopped into conglomerates and then pressed onto a current collector. Instead, the inherently rigid nature of the metal-containing cathode active material of the present invention provides the necessary interstitial space, even after press lamination of the cathode structure onto the current collector, for intercalation of the metal ions produced by oxidation of the anode during electrochemical discharge. Thus, the present cathode material is pressable into a cathode structure without the need for the solvent to serve as a propant to prevent destruction of the cathode material's permeability and without having to impart permeability to the cathode material in a comminuting step.